Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. Further, various valves and/or sensors in the emission control systems may need to be intermittently diagnosed for degradation.
Emission control system diagnostic routines may rely on naturally generated pressure and/or vacuum during diurnal cycles, for example. One example approach is described in U.S. Pat. No. 8,353,273 to McClain. In McClain, a diurnal control valve coupling a fuel tank to a fuel vapor canister is opened and a pressure in a canister vent line is correlated to changes in fuel tank pressure to determine if degradation of the emission control system is present.
However, the inventors herein have recognized an issue with the above approach. During certain conditions, the fuel tank may be equal to atmospheric pressure, and thus there may be no pressure differences to detect within the canister vent line. As such, it may be impossible to differentiate between a degraded fuel tank pressure sensor and a diurnal control valve that is stuck closed.
Accordingly, methods and systems are provided herein to address the above issues. One example method for a fuel emissions system of a hybrid vehicle comprises during a refueling event, maintaining a fuel vapor canister vent line restriction and indicating degradation of a fuel tank pressure sensor based on a pressure in the canister vent line.
In this way, pressure that builds as a result of a fuel tank refueling may be used to correlate pressure in the canister vent line with pressure measured by a fuel tank pressure sensor. If the pressures measured in the canister vent line and fuel tank do not correlate (e.g., if one pressure does not change while the other pressure changes), degradation of the fuel tank pressure sensor, canister vent line pressure sensor, or fuel tank isolation valve may be determined. By doing so, emission control system integrity may be monitored even during conditions where fuel tank pressure is equal to atmospheric pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.